P. Rivas

Hydraulic properties of montmorillonite-quartz and saponite-quartz mixtures

Abstract The work presented in this paper is part of a project of characterization of Spanish clays to be used as backfill and sealing materials in high-level radioactive waste repositories. Hydraulic conductivity and swelling pressure tests have been carried out in montmorillonite-quartz and saponite-quartz mixtures with a quartz weight percentage between 0 and 60%. The linear dependence between the logarithm of the hydraulic conductivity and the clay dry density has been shown, with the pure clay samples having the same behaviour as the clay-quartz ones, provided that the percentage of quartz is below a certain quantity. A threshold value of clay dry density has been found, being the trend of the hydraulic conductivity different above and below that value. Swelling pressure seems to account for this fact and for the slightly different behaviour of both clays.

Characterization of Mg-clays from the Neogene of the Madrid Basin and their potential as backfilling and sealing material in high level radioactive waste disposal

Abstract Three types of sedimentary materials are studied, referred to as green, brown and pink clays. They are exploited in a number of 0.25-3 m thick layers and contain magnesium smectites. The smectite content in the green and brown clays averages over 80%, with illite and sepiolite and small quantities of quartz and feldspar. The pink clay composition corresponds to disordered interstratified kerolite-stevensite, with a varying stevensite content (20dash45%). The structural formula of the smectite in the Adsorption, external specific surface and swelling pressure are very similar in the saponitic clays, with, respectively, average values of: 73–75 meq/100 g (CEC), : 160–170 m2/g (SBET) and 4.6-5.28 MPa (Ps) (precompacted at dry density 1.4 g.cm−3). When there is more than 15–20% sepiolite in the green clays, however, the CEC drops, the external specific surface increases and with unvarying density the swelling pressure doubles. This increase in swelling pressure is related to an additional increase in osmotic pressure produced in the external surfaces. This reaches 15–16 MPa in the pink clays with a specific surface of 250–260 m2/g. Maximum values of 19.4 MPa are reached in discrete levels of green clays containing 70% sepiolite and with a specific surface of 287 m2/g. The genesis of these materials is related to weathering processes in the fluviolacustrine environment in the different zones (lake centre and edge, swamp soils and vadose zone), associated with the evolution of reducing conditions and an alkalinity increase in the water table. The kinetics of these processes would appear to be extremely slow, as long periods without sedimentation are required in order to form significant quantities of magnesium clays and especially saponite. Saponitic clays display qualities that were suitable for their possible use as backfilling and sealing material, although the green clays appear to provide better conditions regarding the rehydration properties associated with the exchange complex and their lower carbonate content.

Forward and inverse modelling of multicomponent reactive transport in single and double porosity media

The term ‘double porosity’ is generally used to represent a conceptual model in which the medium is divided into two or more domains that are coupled by an interaction term for modeling flow and solute transport. While double porosity models are available for water flow and conservative transport, there is not much experience in the fourmulation of double porosity models for multicomponent reactive species. Here we analyze the double porosity behavior in a long-term permeation and tracer test performed on a sample of FEBEX compacted bentonite. FEBEX (Full-scale Engineered Barrier EXperiment) is a demonstration and research project dealing with the bentonite engineered barrier designed for sealing and containment of waste in a high level radioactive waste repository. Hydrogeochemical modelling of porewaters indicate that the main geochemical processes controlling the chemistry of the bentonite are acid-base reactions, aqueous complexation, cation exchange, dissolution/ex-solution of CO 2 , and dissolution and precipitation of highly soluble minerals such as calcite, dolomite, chalcedony, and gypsum/anhydrite. All these processes are assumed to take place under equilibrium conditions. Water flux and chemical data of effluent waters were measured during the experiment. Inverse modelling of this experiment has been carried out using single and double porosity models. The measured breakthrough curve of chloride is consistent with a double porosity model. The model reproduces the trends of measured data except for bicarbonate and pH which are affected by uncertainties in the evolution of CO 2 (g) pressures.